Researcher unveils second Samsung Pay vulnerability

Samsung just can't catch a break these days. Its phones are exploding, and so are its washing machines, and now a security researcher has found a second vulnerability in Samsung Pay.

Salvatore Mendoza demonstrated the first vulnerability at Black Hat in August, where he was able to eavesdrop on a payment transaction, generate a token, and use that new token to make an unauthorized purchase in a different location.

Next week, he will be demonstrating the new Samsung Pay vulnerability at the Ekoparty security conference in Buenos Aires, Argentina.

At Black Hat, he showed how an attacker could eavesdrop on the MST transmission that the Samsung phone sends to emulate a magnetic stripe signal and take advantage of a weakness in how the tokens were generated, allowing him to predict new tokens.

"Tokenization is a very useful technique to protect the confidentiality of credit card data," said Frederik Mennes, senior manager of market and security strategy at VASCO Data Security.

"However it only helps if implemented correctly. For instance, it is very important that payment tokens do not reveal any information about the credit card number they are generated for, and that they cannot be predicted."

The new vulnerability that Mendoza has uncovered uses the NFC communication standard.

"NFC is supposed to be more secure," Mendoza said. "But there is a flaw in it."

Both MST and NFC allow the phone to communicate with the payment terminal wirelessly, he said, but the MST is a one-way communication, while the NFC is a two-way protocol where the phone and the terminal communicate with one another.

With the MST vulnerability, a special device was needed, which cost Mendoza about $50 to make.

The new NFC vulnerability can be exploited with no new equipment at all -- just an app.

"It's easier to carry out the new attack," he said.

The way it works is that the crook stands near the checkout counter with a phone running the interception app while a customer tries to make a purchase. The app eavesdrops on the NFC transmission and grabs the authentication token after the legitimate customer approves the purchase with a fingerprint or PIN code but before the purchase goes through.

The customer gets an error message and tries to put the payment through again.

A new token is issued for the second transaction. Meanwhile, the crook gets up to 24 hours to use the stolen token to make a purchase at any location in the world that has NFC-enabled payment terminals. Mendoza said that he's tested it out at a grocery store, but it could also be done at a BestBuy or a Walmart or another store.

Standing around next to payment terminals holding a phone while people make payments could be suspicious.

"You can make a fake terminal to intercept the tokens," Mendoza said.

Based on his previous communications with the company, Samsung knows about the vulnerability, he said.

But he wasn't favorably impressed by Samsung's reaction to the first issue.

As of this writing, Samsung hasn't responded to our requests for comment. It also hasn't yet issued a response.

Previously, the company issued a statement admitting that it was possible to capture tokens at the point of sale, but that it was difficult to do so.

"This skimming attack model has been a known issue reviewed by the card networks and Samsung Pay and our partners deemed this potential risk acceptable given the extremely low likelihood of a successful token relay attack," Samsung said.

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